For seniority - only your current length of service counts:
For annual leave - previous service "may" be counted:13.6
All service within an appropriate grade, whether on permanent or temporary basis, will count for seniority purposes, with the exception of any periods worked on a casual contract, or during a career break or sabbatical. Unpaid leave will count provided there is no break in contract. Service after age 60 will continue to count. However service must be continuous for seniority purposes, with periods prior to any break in contract (due to re-employment, for example) not generally included, unless there are exceptional circumstances applying to particular cases that are agreed at local level.
All periods of service on a temporary or permanent contract (with periods of casual service included only if they run continuously into a temporary or permanent contract), except for those on unpaid leave, SPPR,or with HM Forces exceeding 15 days, will count towards calculating leave due. Previous periods of PO service may be counted for this purpose.